The universal serial bus (USB) interface is turning into a standard interface for portable devices. However, the original design of USB is targeting PC host. Conventionally, USB is used as an interface between a PC host and a peripheral device. The peripheral devices cannot be connected to one another with USB interface. For example, as shown in FIG. 1, PDA 103, digital camera 104, USB keyboard 105, and USB mouse 106 are all connected to PC host 101 through hub 102a, 102b. 
The Universal Serial Bus Implementers Forum (USB-IF) chose the client-server architecture in defining the USB architecture. That is, the more difficult and complicated processing are left to the server, i.e., PC, while keeping the clients, i.e., the peripherals, as simple as possible. The former is called USB host, and the latter is called USB device. However, as more and more USB devices are developed and the functionality of portable electronic devices improves, it becomes restrictive for some USB devices to stay connected to a USB host. Therefore, USB-IF, based on the original USB specification, defines a USB inter-connection standard, called On-The-Go (OTG).
USB OTG is not independent of USB2.0. Instead, USB OTG provides a way for the USB device to break off from USB host. The USB OTG 1.0 specification provides a concept of Dual-Role device. In other words, an OTG device can be used as a host in addition to as a device. Hence, a USB device can be used in conjunction with a USB OTG device to form a point-to-point client-server connection.
The USB OTG device uses a mini-AB connector, and an ID pin is added to the interface. The ID pin is used to identify which one of the two connected USB OTG devices is the controller, or the controlled, As shown in FIG. 2, while the Device connects to Min-A Plug, the ID pin 201a is connected to ground and the Device is to be the USB OTG Host. While the other device which connects to Mini_B plug, keep the ID pin 210a floating to be USB OTG Device. In the mean time, USB OTG host 201 supplies power through VBUS pin to USB OTG device 210.
As USB OTG must rely on the ID pin to determine the role each plays in the connection, it is inconvenient for the users who are not familiar with mini-AB connector as the user may confuse the setting of ID pin and the USB OTG devices may not function correctly.